Add payment system

[PurkkaKoodari]

Receiving payments for events is a long-standing issue. Ilmomasiina should, therefore, feature a payment integration.

Potential providers for this include Paytrail and MobilePay, and we can potentially support multiple providers.

One thing that is not immediately clear is whether we can enforce a payment deadline programmatically. At least, we probably shouldn't expire unpaid signups in any relatively short time.

• Neither of the providers mentioned above seem to feature a hard payment expiration time. Paytrail's old API has a soft deadline, but that doesn't limit the completion time of payments.
• Even if there was such a feature, it would mean that users could see signups that appear valid on the signup side, but can't be paid for.

Therefore, we're most likely stuck with manually removing unpaid signups after some time, or potentially doing that with a longer delay. That also means that it might be useful to track "useless" payments (linked to deleted signups) manually, and potentially have a refund system for them.